1

Specific Cationic Photoinitiators for Near UV and visible LEDs:

Iodonium vs. Ferrocenium Structures.

Haifaa Mokbel1, 2, Joumana Toufaily2, Tayssir Hamieh2, Frederic Dumur,3 Damien Campolo,3

Didier Gigmes,3 Jean Pierre Fouassier, Joanna Ortyl*4,*Jacques Lalevée1

1) Institut de Science des Matériaux de Mulhouse IS2M – UMR CNRS 7361 – UHA; 15, rue Jean Starcky, 68057 Mulhouse Cedex – France.

2) Laboratoire de Matériaux, Catalyse, Environnement et Méthodes analytiques (MCEMA-CHAMSI), EDST, Université Libanaise, Campus Hariri, Hadath, Beyrouth, Liban.

3) Aix-Marseille Université, CNRS, Institut de Chimie Radicalaire ICR, UMR 7273, F-13397 Marseille, France.

4) Faculty of Chemical Engineering and Technology, Cracow University of Technology, Warszawska 24, Cracow 31-155, Poland.

e-mails: jacques.lalevee@uha.fr ; jortyl@chemia.pk.edu.pl

Abstract:

Two iodonium salts based on a coumarin chromophore are investigated for

polymerization upon light emitting diode irradiations (LEDs). They work as one-component

photoinitiators. They initiate the cationic polymerization of epoxides (under air) and

vinylethers (laminate) upon exposure to violet LEDs (385 and 405 nm). Excellent

polymerization profiles are recorded. Their efficiency is quite similar to that of a ferrocenium

salt. Interpenetrating polymer networks can also be obtained through a concomitant

cationic/radical photopolymerization of an epoxy/acrylate blend monomer. The light

absorption properties of these new salts as well as the involved photochemical mechanisms

are investigated for the first time through electron spin resonance, laser flash photolysis,

steady state photolysis experiments. Molecular orbital calculations are also used to shed some

light on the initiation mechanisms.

Keywords: iodonium salt, ferrocenium salt, cationic polymerization, photoinitiators, light

emitting diode (LED), interpenetrated polymer network (IPN).

2

Introduction

Over the last decades, there has been a growing interest for the industrial application of

photoinitiated cationic polymerization (CP) reactions (see e.g. in [1,2,3,4]) which have several

advantages over the free radical polymerization (FRP). Epoxides and vinyl ethers that exhibit

a low volatility and a negligible toxicity and lead to polymers with good adhesion and

mechanical properties are the most classical and widely used cationic monomers. In contrast

to FRP, molecular oxygen does not inhibit the polymerization and aerated photosensitive

films can be easily photocured. In a dry environment, however, the termination rate is rather

low, permitting the continuation of the polymerization even if the irradiation is stopped

thereby leading to an interesting dark polymerization reaction.

Today, the Light Emitting Diode (LED) technology appears as an alternative to the

traditional UV lamps used in the UV-Curing technology. LEDs are characterized by strong

advantages: low heat generation, low energy consumption, low operating costs, less

maintenance, long life, portability, compact design, easy and safe handling, and possible

incorporation in robots... They have already found a high potential for e.g. digital inkjet

printing, adhesive curing, curing of thin heat-sensitive plastic foils, spot curing and dentistry.

The performance of LEDs vs. conventional light sources in UV-curing experiments has been

evaluated (mostly for FRP [5]).

Onium salts [1,2,3,4,6,7,8] have been extensively studied as photoinitiators (PI) for CP.

Among them, diaryliodonium and triarylsulfonium are commercially important. They consist

in a cationic moiety and a counter anion [6]. The nucleophilicity of the anion is an important

parameter affecting the performance: high nucleophilic anions prematurely terminate the

cationic chain reaction. Consequently, for the commonly used anions, the polymerization

rates usually follow the order: BF4- < PF6

- < SbF6- [1,2,9,10], ~ (C6H5F)4B

- [11]. A large

variety of many other onium salts has been developed (see examples in ref [1a,1b,2,3,4,12]).

Most structures are characterized by high photolysis quantum yields and are efficient when

the irradiation is carried out using lights in the UV region (230-300 nm) [12]. Extending their

spectral response to visible wavelengths requires the presence of photosensitizers and/or free-

radical photoinitiators [1,2,3,4,12,13]. Rather complex multicomponent photoinitiating

systems were thus successfully proposed e.g. photosensitizer/cationic PI; radical PI/cationic

PI; radical PI/cationic PI/additive (see e.g. in a recent review [2]). On the opposite, the search

of more or less colored structures have led to the design of one-component systems sensitive

to higher wavelengths (see e.g. in [1,2,3,4]) and more recently to onium polyoxometalate salts

[14]. The simple introduction of chromophoric moieties on the iodonium scaffold allows

3

interesting red-shifted absorptions (e.g. using a fluorenone [6,15] or a coumarin [16]). Apart

onium salts, organometallic compounds have been known for a long time [17]. In particular,

ferrocenium salt derivatives possessing various ligands (allowing suitable visible light

absorptions) and different nucleophilic anions are efficient (see examples in [3,17,18]).

The design and development of novel high-performance cationic PIs directly adapted to

light-emitting diodes (LEDs) as irradiation devices (especially near UV and visible LEDs) are

still a challenge and clearly attract a great attention in the photopolymerization field [2,19-21].

In the present paper, we focus on a study of two coumarin-based iodonium salts. Indeed, two-

component systems containing Coumarin-153 or Coumarin-7/diphenyl iodonium

hexafluorophosphate Iod for the CP of oxiranes [22] or 7-diethylamino-3-(2’-benz-

imidazolyl)coumarin/Iod for two-photon polymerization [23] are already known. A recent

report was concerned with the design of novel coumarin moiety containing iodonium salts

[16]: replacing one phenyl moiety by the coumarin scaffold red-shifts the absorption.

Promising results for the CP of epoxides and divinyl ethers have been gained upon exposure

to LED @315 nm and LED @365 nm [16] (the polymerization profile was determined using

a fluorescence probe technique); but no excited state processes and no mechanistic approach

are available. Therefore, herein, we propose to use the two derivatives (P3C-P and P3C-Sb)

shown in Scheme 1, monitor the polymerization profiles of a diepoxide, a divinylether and a

diepoxide/acrylate blend in aerated films under violet LEDs (centered at 385 and 405 nm) by

real time FTIR, exemplify the role of the anion and the dark reaction, investigate the excited

state processes by laser flash photolysis and electron spin resonance spin trapping, understand

the absorption properties and the bond cleavage by molecular orbital calculations, carry out

steady state photolysis and, finally, elaborate a mechanism for the initiation step. The

achieved performance will be compared to that of a ferrocenium salt.

Scheme 1. Investigated compounds.

Experimental Section:

i) Synthesis of the different photoinitiators:

4

P3C-P (GalOrti-7M-P) and P3C-Sb (GalOrti-7M-S) were obtained from (Photo HiTech

Ltd., Poland) and used with the best purity available. The synthesis is described in detail in

[16].

The ferrocenium salt - [(1,2,3,4,4a,8a-η)-naphthalene][(1,2,3,4,5-η)-2,4-

cyclopentadien-1-yl]-iron(l+) hexafluorophosphate(1-) - was synthesized according to the

procedure described in the literature [18b].

ii) Chemical compounds:

Diphenyliodonium hexafluorophosphate (Ph2I+ or Iod) and triethyleneglycol divinyl

ether (DVE-3) were obtained from Sigma-Aldrich and used with the best purity available

(Scheme 2). (3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX;

Uvacure 1500) and trimethylolpropane triacrylate (TMPTA) were obtained from Allnex

(Scheme 2). EPOX, DVE-3 and TMPTA were selected as representative monomers: these

monomers are well known in the photopolymerization field and represent excellent structures

to evaluate the initiating ability of the new photoinitiating systems.

3

PF6

DVE-3

_

EPOX Iod (salt) TMPTA

Scheme 2. Chemical structures of the used compounds.

iii) Irradiation sources:

Several lights were used: i) polychromatic light from a halogen lamp (Fiber-Lite, DC-

950 - incident light intensity: I0 ≈ 12 mW cm-2; in the 370-800 nm range); ii) monochromatic

light delivered by LED@385 nm (M385L2- ThorLabs; I0 ≈ 25 mW/cm2 or Hamamatsu : 500

mW/cm2), LED@405 nm (M405L2 - ThorLabs; I0 ≈ 100 mW/cm2), blue LED@455 nm

(M455L3 - ThorLabs; I0 ≈ 80 mW/cm2) and LED@530 nm (ThorLabs; I0 ≈ 60 mW/cm2). The

emission spectra for these irradiation devices are given in [20].

iv) Cationic Photopolymerization:

The experimental conditions are given in the Figure captions. The weight content for the

PI is related to the monomer. The photosensitive formulations (25 µm thick) were deposited

5

on a BaF2 pellet under air for irradiation with different light sources under air. The evolution

of the epoxy group (for EPOX) or vinyl ether (for DVE-3) contents were continuously

followed by real time FTIR spectroscopy (JASCO FTIR 4100) at about 790 cm-1 and 1620

cm-1, respectively [24].

v) Synthesis of Interpenetrated Polymer Networks (IPNs):

The synthesis of interpenetrated polymer networks (IPN) based on EPOX/TMPTA

blend (50%/50% w/w) was carried out in laminate. The evolution of the epoxy group and

double bond content of the film (25 µm thick) was continuously followed by real time FTIR

spectroscopy (JASCO FTIR 4100) at about 790 and 1630 cm-1, respectively as in [24].

vi) Computational Procedure:

Molecular orbital calculations were carried out with the Gaussian 03 suite of programs

[25]. The HOMO and LUMO of the different compounds were calculated with the time-

dependent density functional theory at B3LYP/LANL2DZ level on the relaxed geometries

calculated at UB3LYP/LANL2DZ level. The C-I bond dissociation energy was calculated

from the fully optimized reactants and the free radicals generated.

vii) ESR spin trapping (ESR-ST) experiments:

The ESR-ST experiments were carried out using an X-Band spectrometer (MS 400

Magnettech). The radicals were produced at RT upon the LED@385nm exposure under N2

and trapped by phenyl-N-tbutylnitrone (PBN) according to a procedure described in detail in

[26]. The ESR spectra simulations were carried out with the PEST WINSIM program.

viii) Laser Flash Photolysis

Nanosecond laser flash photolysis (LFP) experiments were carried out using a Q-

switched nanosecond Nd/YAG laser (λexc = 355 nm, 9 ns pulses; energy reduced down to 10

mJ) from Continuum (Minilite) and an analyzing system consisted of a ceramic xenon lamp, a

monochromator, a fast photomultiplier and a transient digitizer (Luzchem LFP 212) [27].

Results and Discussion:

1/ Absorption properties of the photoinitiators:

The absorption spectra of the three salts in acetonitrile are depicted in Figure 1. Their

absorption maxima as well as their molar extinction coefficient ε are compared to those of the

commercial iodonium salt (Iod) in Table 1. Remarkably, the maxima for P3C-Sb, P3C-P, and

DC 1200 are located in the near-UV visible region. As known, Iod does not significantly

6

absorb light above 300 nm [6]. When one of the phenyl substituent of Iod is changed for a

coumarin chromophore, the corresponding absorption spectra are shifted to longer

wavelengths i.e. the maximum absorption being close to 350 nm for P3C-Sb and P3C-P vs.

230 nm for Iod. Compared to the iodonium salts, DC1200 exhibits lower molar extinction

coefficients in the UV but the absorption spreads in the 400-550 nm range. Interestingly, the

absorption spectra exhibit an interesting overlapping with the emission spectra of the

LED@385 and LED@405 nm (for P3C-Sb, P3C-P and DC 1200) and the LED@455 nm and

LED@530 nm (for DC 1200 only).

Figure 1. UV-visible absorption spectra of the different salts in acetonitrile. Insert: absorption

of DC1200 (expanded scale).

200 300 400 500

0

10000

20000

30000

εε εε (M

-1 c

m-1)

λλλλ (nm)

7MP - 009 7MS - 008 DC 1200

P3C-Sb

P3C-P

DC1200

350 400 450 500 550 6000

400

800

1200

1600

2000

λ (nm)

ε(M

-1cm

-1)

200 300 400 500

0

10000

20000

30000

εε εε (M

-1 c

m-1)

λλλλ (nm)

7MP - 009 7MS - 008 DC 1200

P3C-Sb

P3C-P

DC1200200 300 400 500

0

10000

20000

30000

εε εε (M

-1 c

m-1)

λλλλ (nm)

7MP - 009 7MS - 008 DC 1200

P3C-Sb

P3C-P

DC1200

350 400 450 500 550 6000

400

800

1200

1600

2000

λ (nm)

ε(M

-1cm

-1)

Table 1. Comparison of the light absorption properties of the three salts (PC3-P, PC3-Sb,

DC1200) vs. Iod.

Photoinitiators λλλλmax (nm) / ε ε ε ε (M-1cm-1)

PC3-P 347/~17 000

PC3-Sb 347/~17 000

Iod 237/~15 000

DC1200 350-490/1100-200

7

The MOs involved in the lowest HOMO → LUMO energy transition are depicted in

Figure 2. In Iod, the HOMO and LUMO are localized on the phenyl rings. The absorption

red-shift of P3C-P (or PC3-Sb) vs. Iod is due to an enhanced π electron delocalization on the

coumarin moiety as well as to the presence of a charge transfer transition (i.e the highest

HOMO strongly involves the coumarin moiety whereas the LUMO is located in the C-I bond

and presents an antibonding character.

Figure 2. Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) of the iodonium salts. At UB3LYP/LANL2DZ level; isovalue: 0.04.

Iod Coumarin-based iodonium salt

LUMO

HOMO

2/ Cationic photopolymerization of a diepoxide:

Typical conversion-time profiles for the CP of EPOX under air are depicted in Figure 3.

The final conversions (FC) after 800s of irradiation are given in Table 2. The well known Iod

used as a reference is completely ineffective, which is obviously due to its extremely poor

absorption under the LEDs used here. By contrast, upon exposure to the LED@405nm

(Figure 3A), P3C-Sb appeared as an excellent PI: a high conversion (69%) together with a

high rate of polymerization is reached. P3C-P is less efficient (FC = 50%; as expected, this is

due to the nucleophilicity difference of the anion) and behaves as DC1200 (FC = 47%). Upon

the LED@385nm, at low intensity (25 mW/cm2), almost similar FCs are obtained (Figure 3B,

Table 2). Under a higher intensity (500 mW/cm2), the polymerization rates are much higher

I+

O

I+

OO

8

but the FCs are not noticeably changed (Figure 3C). Tack free coatings for the three PIs are

obtained whatever the irradiation devices. Interestingly, a photobleaching of the polymer

films is noted, leading to final colorless coatings. The ferrocenium salt DC1200 can also

initiate CP under blue (455 nm) and green (530) LEDs (Table 2 and Figure 1 in SI). For DC

1200, the FC decreases (from 51 to 39-42%) as a function of the irradiation wavelengths in

the series: 385 nm > 405nm > 530 nm ~ 455nm is in line with the absorption spectrum of

DC1200.

Figure 3. Photopolymerization profiles of EPOX under air; upon the exposure to (A) LED @405nm, in the presence of: (1) P3C-P (1 wt%), (2) P3C-Sb (1 wt%), (3) DC 1200 (1 wt%); (B) LED@ 385 nm in the presence of: (1) P3C-P (1 wt%), (2) P3C-Sb (1 wt%), (3) DC 1200 (1 wt%), and (C) LED @ 385 nm (I = 500 mW/cm2) in the presence of: (1) P3C-P (1 wt%), (2) P3C-Sb (1 wt%).

0 200 400 600 8000

10

20

30

40

50

60

70

80

3

2

1

Co

nver

sio

n (%

)

Time (s)

0 200 400 600 8000

10

20

30

40

50

60

70

32

1

Co

nver

sion

(%)

Time (s)

0 50 100 150 2000

10

20

30

40

50

60

2

1

Co

nver

sion

(%

)

Time (s)

(A) (B)

(C)

Table 2. EPOX final conversion (FC) obtained under air upon exposure to different LEDs for 800s in the presence of the four different PIs. For the LED@385nm operating at 500 mW/cm², the conversions are given for 200 s of irradiation.

LED (385nm) LEDs PIs

I = 25mW/cm2 I = 500mW/cm2

405 nm 455nm 530nm P3C-P (1 wt%) 47% 39% 50%

9

P3C-Sb (1 wt%) 55% 55% 69% npa npa DC 1200 (1 wt%) 51% 47% 39% 42%

Iod (1 wt%) npa npa npa npa npa ano polymerization (<5%).

Concentration effects of the PIs have also been carried out under the LED@405nm

exposure. P3C-Sb appears to be excellent PIs whatever the concentration (the change of FC is

insignificant: ∼2%; Figure 4A). On the opposite, a strong effect is observed when using P3C-

P. Using [P3C-P] = 1%wt and 2%wt, the FC dramatically increases: FC = 50% (Figure 4B

curve 2 vs. curve 1) and FC = 65% (Figure 4B curve 3 vs. curve 2), respectively, instead of

FC = 37%) for [P3C-P] = 0.5%wt. In both cases, as a result of a higher amount of absorbed

light, the improvement on the polymerization rate is noticeable. Tack free coatings are still

obtained.

Figure 4. Photoinitiator concentration effects. Photopolymerization profiles of EPOX under air; upon the exposure to LED @405 nm in the presence of: (A) P3C-Sb (1) 0.5 wt%; (2) 1 wt%; (3) 2 wt%; (B) P3C-P (1) 0.5 wt%, (2) 1 wt%, (3) 2 wt%.

0 200 400 600 8000

20

40

60

80

100

1

23

Con

vers

ion

(%)

Time (s)

0 200 400 600 8000

10

20

30

40

50

60

70

80

3

2

1

Co

nve

rsio

n (%

)

Time (s)

(A) (B)

3/ Dark polymerization:

The presence of a possible dark polymerization after a short irradiation period can be an

advantage of CP compared to FRP [1,28]. Indeed, the reaction can continue through a living

process. The dark polymerization efficiency which is an important parameter for the global

performance of an initiating system decreases here in the series P3C-Sb >> DC1200 ~ P3C-P.

Under irradiation at 385nm or 405nm, using DC1200 or PC3-P, the dark polymerization has a

low efficiency (< 5 %; Figure 5B, Figure 5A). On the opposite, with P3C-Sb, the

polymerization proceeds after the light it switched off (200s of irradiation): FC increase =

10

18% in the dark (after 1800s). This better behavior is also ascribed to the lowest

nucleophilicity of its counter anion.

Figure 5. Photopolymerization profiles of EPOX under air: (A) under LED @405nm exposure (1) P3C-Sb (1%wt), (2) P3C-P (1%wt), and (B) in the presence of DC 1200 (1%wt) (1) LED@385nm, (2) LED @405nm.

0 500 1000 1500 20000

10

20

30

40

50

60

70

Light switched off

2

1

Co

nve

rsio

n (

%)

Time (s)0 500 1000 1500 2000

0

10

20

30

40

50

Light switched off

21

Co

nve

rsio

n (

%)

Time (s)

(A) (B)

0 500 1000 1500 20000

10

20

30

40

50

60

70

Light switched off

2

1

Co

nve

rsio

n (

%)

Time (s)0 500 1000 1500 2000

0

10

20

30

40

50

Light switched off

21

Co

nve

rsio

n (

%)

Time (s)

(A) (B)

0 500 1000 1500 20000

10

20

30

40

50

60

70

Light switched off

2

1

Co

nve

rsio

n (

%)

Time (s)0 500 1000 1500 2000

0

10

20

30

40

50

Light switched off

21

Co

nve

rsio

n (

%)

Time (s)

(A) (B)

4/ Synthesis of Interpenetrating Polymer Networks (IPNs):

The synthesis of interpenetrating polymer networks (IPN) from EPOX/TMPTA blends

(50%/50% w/w) in laminate upon the LED@405nm exposure is feasible (Figure 6). The FCs

of EPOX and TMPTA are not significantly affected by the kind of iodonium salt (47% vs.

44% in Figure 6A curve 1 vs. curve 2 and 37% vs. 32% in Figure 6B curve 2 vs. curve 1).

Tack free coatings are obtained.

Figure 6. Photopolymerization profiles of the TMPTA/EPOX blend (50%/50% w/w) in laminate upon a LED@405nm exposure, using (1) P3C-P (1% wt); (2) P3C-Sb (1% wt); (A) epoxy conversions and (B) acrylate conversions.

11

0 200 400 600 8000

10

20

30

40

50

60

21

Con

vers

ion

(%)

Time (s)

0 200 400 600 8000

10

20

30

40

50

21

Co

nver

sion

(%)

Time (s)

(A) (B)

5/ The cationic photopolymerization of a divinyl ether:

The cationic photopolymerization of DVE-3 was carried out in laminate upon

LED@385 and @405 nm using P3C-P as photoinitiator. Very high polymerization rates as

well as very high final conversions are found ((> 90%; Figure 7); tack free coatings are also

obtained. The polymerization is slightly faster upon LED@385 nm than LED@405 nm due to

the better absorption properties for P3C-P at 385 nm.

Figure 7. Photopolymerization profiles of DVE-3 in laminate upon the exposure to (1) LED @385 nm or (2) LED@405 nm in the presence of P3C-P (1 wt%).

0 200 400 600 8000

20

40

60

80

100

2

1

Co

nve

rsio

n (

%)

Time (s)0 200 400 600 800

0

20

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100

2

1

Co

nve

rsio

n (

%)

Time (s)

6/ Chemical mechanisms

In ESR spin trapping experiments (Figure 8(A) and (B)), phenyl radicals Ph● are clearly

observed during the photolysis of P3C-P and P3C-Sb (hyperfine coupling constants: aN = 14.1

G; aH = 2.2 G, for the Ph●/PBN adducts; reference values in [24a,26]). These Ph● radicals

finally result from a cleavage of the C-I bond (reaction (1a) where Ph-I+-coum stands for

12

P3C-P and P3C-Sb). Another C-I bond breaking might be considered (1b). Molecular orbital

calculations (Scheme 3) show that the corresponding bond dissociation energy BDE (C-I)A

and BDE (Ph-I)B are 46.66 and 61.37 kcal.mol-1, respectively. This suggests a rather selective

I-Ph cleavage reaction in full agreement with the detection of only one kind of carbon

centered radical (Ph●) in ESR-ST. In line with the photolysis of iodonium salts studied in

[12], the primary radicals and cation radicals might be formed here through a singlet or a

triplet radical pair where homolytic and heterolytic cleavages occur. Then (as in [12]), out-of-

cage or/and in-cage processes leading to the release of protons should be possible. These

protons initiate the CP.

Ph-I+-coum (hν) → Ph● + +●I-coum (A) (1a)

Ph-I+-coum (hν) → PhI●+ + ●coum (B) (1b)

BDE(C-I)A

BDE(C-I)B

= 45.66 kcal mol-1

= 61.37 kcal mol-1A

B

PhI●+ Coum●

Ph● Coum-I●+

BDE(C-I)A

BDE(C-I)B

= 45.66 kcal mol-1

= 61.37 kcal mol-1A

B

BDE(C-I)A

BDE(C-I)B

= 45.66 kcal mol-1

= 61.37 kcal mol-1A

B

PhI●+ Coum●

Ph● Coum-I●+

Scheme 3. Possible dissociation mechanisms (A vs. B) of the coumarin-based Iod salts calculated at uB3LYP/LANL2DZ level.

Figure 8. ESR-Spin Trapping spectrum of the radical generated in (A) P3C-Sb upon LED@385 nm exposure; (B) P3C-P upon LED@385 nm exposure; and (C) DC 1200 upon UV exposure; in tert-butylbenzene; under N2; experimental (a) and simulated (b) spectra. Phenyl-N-tert-butylnitrone (PBN) is used as spin trap.

13

3440 3460 3480 3500 3520

(b)

(a)

B (G)

3440 3460 3480 3500 3520

(b)

(a)

B (G)

3300 3310 3320 3330 3340 3350 3360 3370 3380

(b)

(a)

B (G)

(A) (B)

(C)

In steady state photolysis (Figure 9; halogen lamp exposure; under air), the ground state

absorption bands of P3C-P and P3C-Sb decreased at ~ 350 nm as resulting from (1a) and

concomitantly, the absorption at 270 nm markedly increases (formation of photoproducts).

Interestingly, the presence of isobestic points show that no side reaction occurs.

Figure 9. Photolysis of P3C-P (A) P3C-Sb (B) and DC1200 (C) in acetonitrile; under air; halogen lamp exposure.

250 300 350 400 4500,0

0,2

0,4

0,6

30 min

0 min

O.D

.

λλλλ (nm)

250 300 350 400 4500,0

0,2

0,4

30 min

0 min

O.D

.

λλλλ (nm)

300 400 5000

2

4

200 s

0 s

O.D

.

λλλλ (nm)

(A) (B)

(C)

14

In laser flash photolysis (LFP) experiments on e.g. P3C-Sb, a strong bleaching of the

ground state absorption is found at 350 nm demonstrating the high photosensitivity of this

compound (Figure 10). The very short depletion time (<< 10 ns) is limited here by the

resolution time of the device. A weak transient is observed with a maximum absorption in the

500-600 nm range: it can be ascribed to the iodinium radical cation generated after C-I

cleavage process as already noted in the LFP of Iod [29].

Figure 10. (A) Transient absorption recorded at 350 nm for a laser excitation at 355nm for P3C-Sb in nitrogen saturated acetonitrile. (B) UV-vis spectra recorded before and after laser excitation.

0 20 40 60 80-0,005

-0,004

-0,003

-0,002

-0,001

0,000

0,001

0,002

∆∆ ∆∆O

.D.

Time (µµµµs)

200 250 300 350 400 450 5000,0

0,2

0,4

0,6

0,8

1,0

O.D

.

λλλλ (nm)

specre avant LFP spectre apres LFP-- Before LFP-- After LFP

B

A

0 20 40 60 80-0,005

-0,004

-0,003

-0,002

-0,001

0,000

0,001

0,002

∆∆ ∆∆O

.D.

Time (µµµµs)0 20 40 60 80

-0,005

-0,004

-0,003

-0,002

-0,001

0,000

0,001

0,002

∆∆ ∆∆O

.D.

Time (µµµµs)

200 250 300 350 400 450 5000,0

0,2

0,4

0,6

0,8

1,0

O.D

.

λλλλ (nm)

specre avant LFP spectre apres LFP-- Before LFP-- After LFP

200 250 300 350 400 450 5000,0

0,2

0,4

0,6

0,8

1,0

O.D

.

λλλλ (nm)

specre avant LFP spectre apres LFP-- Before LFP-- After LFP

B

A

In the case of DC 1200, cyclopentadienyl (Cp●) radicals are observed upon irradiation

in ESR experiments (Figure 8C; hyperfine coupling constants: aN = 14.3 G; aH = 2.6 G;

reference value in [30]). They are attributed to a Fe-Cp bond dissociation as already proposed

[31]. Under identical light irradiation conditions, DC 1200 exhibits a faster bleaching than the

two iodonium salts. The decrease of the absorption at 280 nm is accompanied by the

15

appearance of two new peaks at 280 nm and 350nm ascribed to the formation of naphthalene

(Figure 9C; isobestic point; no side reaction). As known [18e], the ring opening

polymerization of an epoxide in the presence of such ferrocenium salts is assumed to take

place through a ligand transfer reaction where the arene moiety is replaced by three molecules

of epoxide.

Conclusion:

In the present paper, we have proposed two modified iodonium salts for exposure to

violet LEDs. They work as PIs for the ring opening polymerization of an epoxide under air

and exhibit an efficiency similar to that of a ferrocenium salt; interpenetrating polymer

networks can also be synthesized; the photopolymerization of divinylethers is also easily

achieved. Their behavior as one-component photoinitiators is expected to be of interest, the

use of a photosensitization process (that is always tricky) becoming unnecessary. Other

iodonium salts applicable as high-performance PIs operating under blue or green LEDs

deserve to be developed.

Supporting Information:

This information is available free of charge via the Internet.

16

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19

Graphical Abstract:

BDE(C-I)A

BDE(C-I)B

= 45.66 kcal mol-1

= 61.37 kcal mol-1A

B

PhI●+ Coum●

Ph● Coum-I●+

BDE(C-I)A

BDE(C-I)B

= 45.66 kcal mol-1

= 61.37 kcal mol-1A

B

BDE(C-I)A

BDE(C-I)B

= 45.66 kcal mol-1

= 61.37 kcal mol-1A

B

PhI●+ Coum●

Ph● Coum-I●+

Cationic Polymerization: epoxy, vinylether

Interpenetrated Polymer Network Synthesis (IPN)

Specific Cationic Photoinitiators for Near UV

and visible LEDs: Iodonium vs. Ferrocenium

Structures.

BDE(C-I)A

BDE(C-I)B

= 45.66 kcal mol-1

= 61.37 kcal mol-1A

B

PhI●+ Coum●

Ph● Coum-I●+

BDE(C-I)A

BDE(C-I)B

= 45.66 kcal mol-1

= 61.37 kcal mol-1A

B

BDE(C-I)A

BDE(C-I)B

= 45.66 kcal mol-1

= 61.37 kcal mol-1A

B

PhI●+ Coum●

Ph● Coum-I●+

Cationic Polymerization: epoxy, vinylether

Interpenetrated Polymer Network Synthesis (IPN)

Specific Cationic Photoinitiators for Near UV

and visible LEDs: Iodonium vs. Ferrocenium

Structures.